Implantable medical systems that are designed to deliver electrical stimulation, for example, to cardiac muscle or the spinal cord, and/or to monitor bodily electrical activity, typically include a relatively compact implantable device to which one or more elongate implantable electrical leads are coupled, for example, like the exemplary prior art system 10 schematically shown in FIG. 1A. FIG. 1A illustrates system 10 including an implantable defibrillator device 500 and a defibrillation lead 100, which is connected to device 500 and extends transvenously therefrom, into a heart of a patient, such that a defibrillation electrode 11 and a pace-sense electrode 13 of lead 100 are located in the right ventricle of the heart. Those skilled in the art appreciate that a power source and circuitry of device 500 are contained in a hermetically sealed housing 55 of device 500, which housing 55, being formed from a conductive metal such as titanium, may function as an electrode, in concert with electrode 11, to deliver high voltage pulses for defibrillation therapy in response to a cardiac arrhythmia, for example, sensed by electrodes 13, 11.
With reference to FIG. 1B, an outer insulation sheath 12 of lead 100 contains a first elongate conductor 20 that couples electrode 11 to a contact 151 of a connector terminal 15 of lead 100, and a second elongate conductor 135, which is isolated from first conductor 20 and couples electrode 13 to a contact pin 153 of terminal 15. FIG. 1A further illustrates device 500 including a connector module 51 that has a port 501 into which connector terminal 15 is inserted for electrical coupling with the circuitry contained in housing 55, for example, via electrical contacts, which are mounted within port 501 and coupled to the circuitry via hermetically sealed feedthroughs. Suitable constructions for such a connector module and lead connector are known to those skilled in the art.
With further reference to FIGS. 1A-B, electrode 11 is shown formed by a coiled conductor wire, which may be an exposed distal portion of conductor 20 or a separately formed coil coupled to conductor 20, for example, by a weld or a crimp joint. Such electrodes were at one time made entirely of from a platinum-iridium alloy (Pt—Ir), since Pt—Ir provides an effective bio-stable, bio-compatible, and corrosion-resistant electrode surface interface. More recently, in order to reduce costs, such coil electrodes have been made of a composite structure employing a tantalum core provided with an overlay or cladding of platinum-iridium. One such construction is disclosed in U.S. patent application Ser. No. 13/664,782, filed Oct. 31, 2012 by Boser, et al. and incorporated herein by reference in its entirety.